in the generations. Generation 1 uses a two-pin frame, generation 3 and later use three-pin frames. The breech face and extractor changed with the Gen 2 also, I think. Gen 3 added a light rail and contoured finger grooves, and I think Gen 4 added new serrations, a different mag release, and adjustable backstraps.

"I have always been a soldier. I have known no other life. The calling of arms, I have followed from boyhood. I have never sought another." From The Virtues of War, by Steven Pressfield.